1. Field of the Invention
The present invention relates to a method of soldering a bump of a bumped work to a pad of another work, and, more particularly, to a method of soldering a bump of a bumped electronic component on a pad of a circuit board.
2. Description of the Prior Arts
Soldering is conventionally performed between a bumped electronic component such as a flip chip and a circuit board such as a printed circuit board as follows. First, flux is applied on the surface of a bump formed on the bumped electronic component, or of a solder portion formed on the pad of the circuit board. Then, the bump is placed on the solder portion to mount the bumped electronic component on the circuit board. Subsequently, the circuit board is fed into a heating furnace to heat the solder portion to a temperature at its melting point (typically, about 200.degree. C.) or higher for melting it, and then to cooling the solder portion so as to solidify it. In this procedure the bumped electronic component is mounted on the circuit board. Thereafter, a conductivity test is conducted for examining whether or not the bump of the bumped electronic component is properly soldered on the pad of the circuit board. The conductivity test is typically performed by attaching a probe of the tester to a terminal of the circuit board to check the conductivity. If the results of the conductivity test are not acceptable, a repair work is performed on the bumped electronic component. In the repair work, the circuit board mounted with the bumped electronic component is heated to remelt the solder portion, thereby removing the bumped electronic component from the circuit board. Once the bumped electronic component is removed, a part of the solder portion remains affixed to the pad. Then, the solder portion on the pad is scraped off with a tool or the like. After the solder portion is scraped off, a solder portion of a suitable amount is formed again on the pad. When the correction of the circuit board is completed with the above procedure, the bumped electronic component is mounted again on the circuit board.
However, a problem associated with the conventional method is that the remolten solder on the solder portion flows sidewise, and adjacent solder portions connect each other to cause the formation of a solder bridge, leading to short-circuiting. The reason therefor lies in the following.
Since the solder portion (alloy of low melting point metals such as Pb and Sn, i.e. lead and tin) formed on the pad of the circuit board is easily oxidized when it contacts air, an oxide film has been formed on the surface of the solder portion. The oxide film hinders wettability of the solder, and causes defective conductivity between the bump and the pad on the circuit board.
Then, the wettability of solder is conventionally improved by applying flux on the bump or solder portion. When the flux is used, the oxide film can be removed in heating and melting the solder portion, whereby the wettability of solder is improved to firmly solder the bump on the pad. However, if the oxide film is removed by using the flux, the fluidity of solder increases, so that adjacent solder portions connect each other to cause a solder bridge.
Since the flux has properties which deteriorate a circuit pattern on the circuit board, the conventional soldering method requires a step for washing and removing the flux after the bump is soldered on the pad. Thus, another problem associated with the conventional method is the number of steps required.
Furthermore, another problem associated with the conventional method for conducting a repair is that it is complicated, which requires much labor and a number of steps.